1. Field of the Invention
The present invention relates to an image sensing apparatus that has a plurality of focus detection areas and performs exposure control using the defocus amounts of the plurality of focus detection areas.
2. Description of the Related Art
Some image sensing apparatus such as a digital camera and digital video camera have an auto-focus function of determining whether an object is in focus, and driving a focus lens to focus on the object. Auto-focus methods include a phase difference detection method. In the phase difference detection method, the object image is divided by pupil division, obtaining two images. The defocus amount of the object is detected from the phase difference between the two images. Based on the detected defocus amount, the focus lens in the photographing optical system is moved to focus on the object.
Some image sensing apparatus have an auto-exposure function of measuring the brightness of an object and determining a correct exposure when sensing an object image. The exposure is determined based on the photometric value of an object and the sensitivity (or equivalence) of the image sensing apparatus. Upon determining the exposure, the aperture and shutter speed are changed in accordance with a necessary exposure, and an image can be sensed with a correct exposure.
When the image sensing apparatus has both the auto-focus and auto-exposure functions, it measures the brightness of an object in an in-focus object image area detected by the auto-focus function. The image sensing apparatus can focus on the object and sense its image with a correct exposure. Japanese Patent Laid-Open No. 2001-356384 discloses a technique of obtaining a correct exposure. In this technique, weightings are determined in accordance with the defocus amounts of a plurality of focus detection areas. The weighted average of the photometric values of the focus detection areas is calculated using the weightings, thereby calculating the photometric value of all the areas.
However, when the technique of obtaining a correct exposure in Japanese Patent Laid-Open No. 2001-356384 is applied to continuous shooting, the continuous shooting speed may decrease because the calculation of defocus amounts takes time. To prevent this, priority may be given to the continuous shooting speed, and image sensing may be done regardless of whether the calculation of defocus amounts has been completed. However, among images obtained in continuous shooting, the exposure may differ between an image obtained by performing exposure control using defocus amounts and one obtained by performing exposure control without using defocus amounts.